1. Field of the Invention
The present invention relates to a novel mesomorphic compound, a liquid crystal composition containing the compound. The present invention also relates to a liquid crystal device using the composition and a display apparatus. In particular, the present invention relates to a liquid crystal which provides a liquid crystal composition with improved responsiveness to an electric field, as well as to a liquid crystal device for use in a liquid crystal display apparatus, a liquid crystal-optical shutter, etc.
2. Related Background Art
Hitherto, liquid crystal materials have been used in various fields as an electro-optical device. Most liquid crystal devices which have been put into practical use have employed twisted nematic (TN) type liquid crystals, as shown in "Voltage-Dependent Optical Activity of a Twisted Nematic Liquid Crystal" by M. Schadt and W. Helfrich "Applied Physics Letters" Vol. 18, No. 4 (Feb. 15, 1971) pp. 127-128.
These TN devices are based on the dielectric alignment effect of a liquid crystal in which the average molecular axis direction of the liquid crystal molecule is directed to a specific direction in response to an applied electric field because of its dielectric anisotropy. However, the accepted lower limit of response speed of such TN materials is on the order of milli-seconds, which is too slow for many uses. Additionally, while a simple matrix system of driving is considered to be the most promising for use as a large-area flat display considering such cost, productivity, etc. The matrix system uses an electrode arrangement for multiplex driving wherein an address signal is sequentially, periodically and selectively applied to scanning electrodes in synchronism with prescribed data signals which are selectively applied in parallel to signal electrodes the address signal.
However, when the TN liquid crystal is used with a matrix driving system, an electric field is applied to certain regions called "half-selected points", i.e., those regions where a scanning electrode is selected and signal electrodes are not selected, or regions where a scanning electrode is not selected and a signal electrode is selected. TN display devices operate normally when the difference between a voltage applied to the selected points versus a voltage applied to the half-selected points is sufficiently large, and the threshold voltage level required for aligning liquid crystal molecules perpendicularly to an electric field is between the values of the voltages of the selected and half selected pants. However, as the number (N) of scanning lines increases, a time (duty ratio) during which an effective electric field is applied to one selected point decreases with a ratio of 1/N when an image area (corresponding to one frame) is scanned. Accordingly, as the number of scanning lines increases, the voltage difference of an effective value applied to selected and non-selected points when scanning is effected repeatedly decreases. As a result, drawbacks such as lowered image contrast or occurrence of interference (or crosstalk) occur. These phenomena are regarded as essentially unavoidable when driving liquid crystals which are not bistabile, i.e. liquid crystal molecules which are horizontally or vertically oriented with respect to the electrode surface only when an electric field is effectively applied using a time storage effect. To overcome these drawbacks, the voltage averaging and multiple matrix methods, etc. have been proposed. However, since no method is sufficient to overcome the duty ratio problem, development of display devices with large image areas or high packaging densities is delayed because it is difficult to sufficiently increase the number of scanning lines.
To overcome drawbacks with such prior art liquid crystal devices, display devices using liquid crystal materials having bistability has been proposed by Clark and Lagerwall (e.g. Japanese Laid-Open Patent Appln. No. 56-107216, U.S. Pat. No. 4,367,924, etc.). In this instance, as the liquid crystals having bistability, ferroelectric liquid crystals (FLC) having chiral smectic C (SmC*) or smectic H (SmH*) phase are generally used. These liquid crystals are bistable, e.g., exhibit first and second stable states with respect to an electric filed applied thereto. Accordingly, as different from optical modulation devices in which TN liquid crystals are used, the bistable FLC molecules are oriented to first and second optically stable states with respect to one and the other electric field vectors and retain the resultant state even in the absence of an electric field.
In addition to the above-described characteristic of bistability, FLC also exhibit an excellent high-speed responsiveness because the spontaneous polarization of the FLC and an applied electric field directly interact with each other to induce transition of orientation states. The resultant response speed is faster than the response speed due to the interaction between dielectric anisotropy and an electric field of TN materials by 3 to 4 magnitudes.
Thus, a ferroelectric liquid crystal potentially has excellent characteristics which make possible essential improvements to many of the problems with conventional TN devices. Particularly, the application to a high-speed optical shutter and a display of high density and a large picture is expected. For this reason, there has been made extensive research with respect to liquid crystal materials showing ferroelectricity. However, ferroelectric liquid crystal materials developed heretofore may not sufficiently satisfy necessary characteristics required for a liquid crystal device including low-temperature operation characteristic, high-speed responsiveness, etc. For instance, among response time (.tau.), magnitude of spontaneous polarization (Ps) and viscosity .eta., the following relationship exists: .tau.=.eta./(Ps.E), where E is an applied voltage. Accordingly, a high response speed can be obtained by (a) increasing the spontaneous polarization (b) lowering the viscosity or (c) increasing the applied voltage. However, the driving voltage has a maximum acceptable upper limit since it is driven with an IC, etc., and actually should desirably be as low as possible. Accordingly, it is necessary either to lower the viscosity or increase the spontaneous polarization.
Generally, a ferroelectric chiral smectic liquid crystal having a large spontaneous polarization provides a large internal electric field in a cell and is liable to pose many constraints on the device construction of the device. Further, an excessively large spontaneous polarization is liable to accompany an increase in viscosity, so that a noticeable increase in response speed may not necessarily be attained as a result.
Further, since the actual operation temperature of a display device is generally from about 5.degree.-40.degree. C., over which the response speed changes by a factor of about 20, the change in response speed actually exceeds the range controllable by driving voltage and frequency.
As described hereinabove, commercialization of a FLC device requires a liquid crystal composition assuming a chiral smectic phase which has not only a large spontaneous polarization but also a low viscosity, a high-speed responsiveness and a minimal temperature-dependence of response speed.